Short answer: A UL94 rating is not a property of a material family — it is a test result for one specific grade, at one specific thickness, in one specific color. “FR ABS” or “V-0 nylon” written on a drawing is not a specification. Before you approve any flame-retardant compound, confirm the rating at your actual wall thickness, in your actual color.
That single habit prevents most of the “the mold was already cut when the part failed” disasters.
The UL94 ratings, in plain terms
UL94 classifies how a plastic sample behaves under a small flame: how fast it ignites, whether it self-extinguishes, and whether burning drips fall and ignite something below.
| Rating | Test | What it means |
|---|---|---|
| HB | Horizontal | Basic horizontal burn — the lowest flame classification |
| V-2 | Vertical | Self-extinguishes; flaming drips may occur |
| V-1 | Vertical | Stricter; no drips that ignite the cotton indicator |
| V-0 | Vertical | Fastest self-extinguishing; no qualifying flaming drips |
| 5VB | Vertical, more demanding | Burn-through of the specimen is permitted |
| 5VA | Vertical, most demanding | Burn-through is not permitted |
V-0 is not always the right target. It is commonly specified, but for a non-critical cover, internal guide or low-risk part, V-1, V-2 or even HB may meet the real requirement at lower material cost and less molding risk. Over-specifying V-0 wastes money; the requirement should come from the product standard (IEC 62368-1, IEC 60335, UL 508A, an OEM spec), not habit.
The wall-thickness trap (this is where projects fail)
A UL94 rating is tied to tested specimen thickness. A grade listed as V-0 at 1.5 mm does not automatically pass V-0 at 0.8 mm.
If your housing has a 0.8 mm wall but the grade is only certified V-0 at 1.5 mm, you cannot assume the rating carries over. At thinner sections, the material may fail the test even though the datasheet “looks fine.”
As a rule of thumb:
| Wall thickness | Typical achievable rating |
|---|---|
| Below 0.8 mm | HB in most standard FR grades |
| 0.8–1.5 mm | V-2 or V-1 |
| 1.5–3.0 mm | V-0 (most common range for electronics enclosures) |
| 3.0–6.0 mm | V-0 or 5VB |
| Above 6.0 mm | 5VA |
Match the drawing’s minimum wall thickness (including thin ribs and remote flow areas) to the tested thickness before approving the resin.
Which flame-retardant material fits your part
| Material | Best for | Watch out for |
|---|---|---|
| FR ABS | Cost-sensitive housings, covers, panels | Heat and UV limits; weak weld lines |
| FR PC | High-impact, higher-heat enclosures | Higher cost; molded-in stress cracking |
| FR PC/ABS | Electronic housings, appliance parts | Grade/thickness/color must be checked |
| FR PA66 / PA6 | Connectors, brackets, structural parts | Moisture absorption; dimensional shift |
| FR PBT | Connectors, relays, switches | Some grades brittle; dimensional stability is its strength |
| FR PP | Low-cost electrical/appliance parts | Lower stiffness and heat resistance |
The choice is never just the flame rating — it is impact, heat, CTI, surface, molding behavior and cost together.
Beyond UL94: the specs buyers forget
- CTI (Comparative Tracking Index) — matters for live parts and creepage-distance design. Check the required CTI level (250 / 400 / 600).
- Glow wire (GWFI / GWIT) — required for many appliance parts that sit near a heat source.
- Color. A UL94 rating is tied to the formulation including colorant. A custom pigment can change flame behavior enough to affect the rating. Use a pre-compounded FR grade already certified for your color, rather than color-matching a certified grade afterward.
- Mechanicals. FR additives can reduce impact, stiffness and weld-line strength. Validate drop impact, screw bosses and snap-fits on the real molded part.
How to write the spec so it survives
Instead of “PC, UL94 V-0,” specify:
- Exact commercial grade and supplier
- Required UL94 rating and the tested thickness
- Minimum wall thickness on the drawing
- Color (or confirm the certification covers your color)
- CTI and glow-wire targets if they apply
- Compliance documents needed (RoHS, REACH, UL94, halogen-free statement)
FAQ
Do I always need UL94 V-0? No. The required rating depends on the application, part location and product standard. V-1, V-2 or HB may be sufficient for non-critical parts. Let the real requirement — not habit — set the target.
Can a V-0 grade fail on my part? Yes, most commonly when the molded wall is thinner than the tested thickness, or the color wasn’t part of the certification, or the part cracks around screws under assembly load.
Does changing the color affect the flame rating? It can. Ratings are tied to a specific formulation including pigment. Confirm the certified grade covers your color before tool release.
What’s the difference between UL94 V-0 and glow wire? UL94 measures flame behavior of a small specimen; glow-wire tests (IEC 60695-2) simulate a hot wire touching the part. Many appliance standards require both.
By the FitPoly technical team. Last updated October 2026.
Specify the flame rating, not just the word “V-0”
FitPoly compounds flame-retardant engineering plastics — including FR PC/ABS, FR PA6 and FR PA66 — with grade-specific UL94 documentation, tested thickness and color coverage. Before you release the tool, we’ll help you match the rating to your actual wall thickness and color.
Send your drawing and UL94 target at fitpolymaterials.com/contact.
